A unit adapted to float in a body of water, in particular a unit for drilling for or production of natural resources such as hydrocarbons, is generally provided with a station-keeping system in order to maintain the geographical position of the unit. Such a station-keeping system may comprise one or more mooring lines and/or a dynamic positioning system which in turn comprises a plurality of thrusters.
A unit with a station-keeping system comprising a mooring line generally has a mooring line handling arrangement, comprising inter alia a windlass and/or a chain jack, such that the length of the portion of the mooring line extending between the unit and the sea bed—which portion hereinafter is referred to as the connection portion—may be altered. Such an alteration may for instance be occasioned by the fact that the position of the unit needs to be changed. A tail portion of the mooring line, i.e. a portion of the mooring line which is located on the opposite side of the handling arrangement as compared to the connection portion, is generally stored onboard the unit. For instance, the tail portion may be stored in a chain pipe and/or a chain locker of the unit.
However, there are problems related to the onboard storage of the tail portion. For instance, since the tail portion generally has a substantial weight, the onboard storage will contribute to an increase of the vertical centre of gravity (VCG) of the unit which in turn will impair the load carrying capacity of the unit. Moreover, if the unit is in a damaged condition such that the unit is subjected to a large inclination, there is a risk that the chain pipe and/or chain locker may be filled with sea water which results in an even larger inclination of the unit.
In order to solve the latter problem, US 2005/0022712 proposes the use of a sealing device for providing a seal between the tail portion and a chain pipe. Although the '712 solution in many cases is appropriate in terms of avoiding the additional inclination during a damaged condition, the problem of the increased VCG is not solved by the '712 solution.
As may be realized from the above, there is a need for improvements of units comprising mooring lines.